TOUCH, HAPTICS & PROPRIOCEPTION

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Touch

The oldest perceptual modality

The most social sense

The most closely linked to motion and action

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Receptive Field

• Mechanoreceptors detect skin deformationsTactile acuity is determined by how close the mechanoreceptors are to each other and by the size of the receptive field

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Receptive Field

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Receptive Field

The two-point threshold for any part of the body is determined by the size of the receptive fields and the extent of overlap

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Receptive Field

The two-point threshold for any part of the body is determined by the size of the receptive fields and the extent of overlap

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Types of Fibers

• Rapidly Adapting (RA) -respond to changes in stimulation, but do not continue to respond to constant stimulation

• Slowly Adapting (SA) -respond to constant stimulation• Punctate - small receptive fields with distinct boundaries• Diffuse - large receptive fields with non-distinct

boundaries

RA SA

Punctate RA-Punctate SA-PunctateDiffuse RA-Diffuse SA-Diffuse

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The nerve fibers enervate four receptor types

Receptors

1. Meissner Corpuscles (RA-punctate) responds best to active touch involved in object exploration

2. Pacinian Corpuscles (RA-diffuse) extremely sensitive over a large receptive field -- blow gently on the palm of your hand

3. Merkel Disks (SA-punctate) constant sources of stimulation over a small area, such as if you were carrying a pebble

4. Ruffini Endings (SA-diffuse)constant stimulation over a larger area - also detects skin stretch

• Free nerve endings - pain fibers & thermal conductance fibers

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Four Receptor Types

a) Merkel ReceptorSA Punctate

b) Meissner CorpuscleRA Punctate

c) Ruffini EndingSA Diffuse

d) Pacinian Corpuscle RA Diffuse

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Cross Section of the Skin

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Peripheral Pathways of Touch

• Proprioceptors• Mechanoreceptors• Two pathways for pain (both of which are

independent from other tactile or proprioceptive pathways) –

1. one fast pathway for sharp pain,

2. one slow pathway for dull pain

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Peripheral Pathways for Touch

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Peripheral PathwaysFor Touch

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Cortical Pathways of Touch

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Sensation of Touch

• Adjacent portions of skin surface tend to be represented by adjacent portions of cortex

• Cortical magnification for lips, nose and fingers

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Cortical Magnification

• The receptive fields and cortical representations give more acuity to fingers, mouth, nose and tongue

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Cortical Magnification corresponds to greater acuity

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Cortical Plasticity for Touch

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Faculties of Touch

1) Object identification

2) Proprioception

3) Object localization

4) Detection of tissue damage

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Object Identification

• Haptics provide abstract, 3-D information about object form

• Spatial Frequency Analysis of Skin Deformations provides information about local form texture, density, mass and torque

• Thermal conductance gives information about object material properties

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Haptics

• Active touch is a mode of perceptual explorarion (c.f., visual search)

• Haptics can detect gross features of objects form, mass, weight distribution, torque,

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Haptics

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Spatial Frequencies• Spatial frequencies correspond to the rate of minute

deformation which determine texture (i.e. coarse, smooth etc.)

• Larger deformations correspond to object features• Active touch (Haptics) allows us to determine the

position of tactile features on an object relative to each other

• These features corresponds to visual information about texture, shape and form and relative position

• Sensory Substitution --Braille

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Proprioception

• All muscles have nerve fibers which detect the amount the muscle is stretched

• All joints have fibers which detect the relative position of each bone

• Together these allow you to determine the position of every part of your body.

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Proprioception

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Prioprioception Includes The Vestibular SenseOcular Motor

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Haptics, Proprioception and Object Location

• Prioprioceptors allow you to determine the position of every part of your body.

• Haptic touch is the interaction of proprioceptive and mechanoreceptive information

• Object location is determined (within a narrow range) by the position of the object relative to the body

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Interactions of Touch & Vision

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Thermal Conductance • A uniquely tactile object property • The rate at which heat is gained or lost between

the skin and an object - we do not detect absolute temperature

• Metal objects, fluids etc. create a more extreme sensation of temperature than do other objects (despite no differences in absolute temperature) because heat energy is transferred more easily to and from them

• If a metal and a wooden block are both 150°, the metal block will feel hotter than the wooden block.Likewise for the same blocks at 0° the metal block will feel colder

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PainPain Pathway

Somatosensory Cortex

Thalamus

Spinal Cord

Dorsal Horn

Nerve

Free Nerve Ending

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PainSharp Pain Reflex• Limb is pulled

toward the body out of harms way

• Normal pain information continues to brain for more considered action

Somatosensory Cortex

Thalamus

Spinal Cord

Dorsal Horn

Nerve

Muscle

Free Nerve Ending

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Pain:The Reflex Arc

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Gaiting Pain

• Gate control theory of pain - pain is actively suppressed in emergency situations by messages sent from the brain to the

• Dorsal Horn• This allows you to

escape on a broken limb or with a gash

• Pain resumes when emergency is over

Cortex

Thalamus

Spinal Cord

Dorsal Horn

Nerve

Free Nerve Ending

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Gating Pain: Dorsal Horn (Root)

Back

Chest

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Spino-Thalamic Pathway: Temperature & Pain

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Medial Lemniscal Pathway: Mechanoreceptors & Proprioception

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Phantom Pain

1. After surgical removal of a limb, sensations resume in the limb

2. In 90% of patients, the sensations are very painful

3. In 60% the pain is excruciating: described sometimes as an arm on fire, being torn or punctured, great pressure

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Phantom Pain: Strange Facts

1.Stimulating certain areas of skin (e.g., face) may aggrevate phantom pain.

2.Severing the nerve doesn’t help. Blocking the nerve doesn’t help. Removing the portion of the thalamus that relays the information to the brain doesn’t help!

3.Stimulating the nerve does help. Electric or manual stimulation of the stump helps tremendously electric more so).

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Phantom Pain: A Theory

1. Recall that the cortex is plastic and may reorganize.

2. Normally this involves annexing juvenile or unused neurons (indicated by low activity level)

3. In amputation the entire area of say an arm is no longer active in the brain

4. Other areas attempt to annex these neurons5. Because the neurons already had a specialization

(e.g., sharp pain) and are no being stimulated by adjacent areas of cortex, the subject feels pain.

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The cortical areas for the face annex the cortical areas for the arm and fingers.

Some of those neurons were previously specialized for pain.

Phantom Pain

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Phantom PainTENS (transcutaneous electrical stimulation) uses tiny electrical pulses, delivered through the skin to nerve fibers, to directly stimulate nerves in the stump that formerly enervated the limb.Spinal cord stimulation uses electrodes surgically inserted within the epidural space of the spinal cord. Deep brain or intracerebral stimulation is considered an extreme treatment and involves surgical stimulation of the brain.These treatments all create activity in the cortical region associated with the former limb, which prevents adjacent neurons from annexing